Communication module

ABSTRACT

A communication module that may be smaller in size but capable of providing a high-rate communication with an excellent high frequency characteristic includes a semiconductor laser (LD) ( 10 ); a flexible printed circuit board (FPC) ( 11 ) on which the LD ( 10 ) is mounted and to which the LD ( 10 ) is electrically connected; a stem ( 12 ) through which the FPC ( 11 ) is inserted and to which the FPC ( 11 ) is then fixed; and a cap ( 13 ) so disposed as to cover the LD ( 10 ). The FPC ( 11 ) is used, instead of lead pins, to supply a power to the LD ( 10 ), derive signals therefrom and so on, whereby the high frequency characteristic can be improved. In addition, employment of a package structure comprising the stem ( 12 ) and cap ( 13 ) realizes the smaller size.

TECHNICAL FIELD

The present invention relates to a communication module that has apackage structure comprising a stem and a cap, and in particular, to acommunication module that exhibits an excellent high frequencycharacteristic, allows a high rate communication to be performed, andthat is the most suitable for optical communications.

BACKGROUND ART

As a communication module conventionally used for opticalcommunications, one is known whose structure is generally called CANtype package, for example. FIG. 6 (A) is a front elevational view of thelongitudinal cross-sectional structure of a conventional opticalreceiver module, and FIG. 6 (B) is a plan view of a stem thereof asviewed from a BOARD connection side. The optical receiver module 100 asshown in FIG. 6 (A) includes a photodiode (PD) 101, the stem 102 bearingthe PD 101, and a cap 104 having, in its top portion, a light-collectivelens 103 and so disposed as to cover the PD 101. The PD 101, which ismounted on a sub-mount 105 fixed on the stem 102, receives, via thelight-collective lens 103, an incident light from an optical fiber 200fixed above the lens 103.

The stem 102 includes a plurality of holes 102 a through whichrespective lead pins 106, which supply a power to the PD 101 and deriveelectric signals therefrom, are inserted. Fixing materials 107, made ofsolder, low-melting glass or the like, are used to fix the respectiveinserted lead pins 106, thereby maintaining hermeticity and mechanicalstrength. Further, wires 108 are used to provide electrical connectionsbetween the PD 101 and its associated lead pins 106 and between thesub-mount 105 and its associated lead pins 106.

FIG. 7 is a side view showing a state in which the conventional opticalreceiver module is connected to the BOARD. As shown in FIG. 7, theoptical receiver module 100 is mounted on the BOARD 110 (i.e., asubsequent-stage circuit board) on which a preamplifier 109, whichamplifies the electric signals from the PD 101, and other electroniccircuit components (not shown) are implemented. Mounting the opticalreceiver module 100 on the BOARD 110 is achieved by bending the leadpins 106 of the optical receiver module 100 and then soldering therespective one ends of the lead pins 106 to corresponding wiringpatterns 111 formed on the BOARD 110. A wire 112 is used to connect awiring pattern 111 to the preamplifier 109, whereby the PD 101 isconnected to the preamplifier 109 through the lead pin 106, wiringpattern 111 and wire 112.

Such optical receiver modules having the structure as described aboveare capable of providing communications of up to 100 Mbps and hence arein widespread use. In recent years, however, communication modules thatare capable of providing communications of higher rates than 100 Mbpsand that are smaller in size have been desired. Achieving such ahigher-rate communication requires an improvement of the high frequencycharacteristics of communication modules. Then, there exists atechnology for reducing the lengths of lead pins to reduce theinductances and capacitances in the previously described structurecalled CAN type package (See Patent Publication 1). Patent Publication 2also discloses a communication module wherein the lengths of lead pinshave been reduced.

Patent Publication 1: Japanese Official Gazette of Patent Laid-OpenPublication No. 2001-196766

Patent Publication 2: Japanese Official Gazette of Patent Laid-OpenPublication No. 2001-298217

DISCLOSURE OF INVENTION

Problem to be Solved by the Invention

There is, however, a problem that the conventional communication modulesemploying the foregoing structure called CAN type package have alimitation of improvement of the high frequency characteristics andhence has a difficulty in achieving higher-rate communications.

As shown in the Patent Publications 1 and 2, a reduction of the lengthsof the lead pins can somewhat improve the high frequencycharacteristics. However, when workability of mounting the communicationmodule on the BOARD is taken into account, it is found that the leadpins need some lengths and hence the improvement of the high frequencycharacteristics as achieved by reducing the lead pin lengths is limited.

The high frequency characteristic can be also improved by changing,instead of the lead pin lengths, the sizes of the fixing materials, madeof solder, low-melting glass or the like, that fix the respective leadpins to the stem. The high frequency characteristic is dependent on thepermittivity E of the fixing materials 107 and on the diameters Rthereof (See FIG. 6 (B)). Specifically, the high frequencycharacteristic can be improved by increasing the ratio of the fixingmaterials 107 to the lead pins 106; that is, reducing the diameters r ofthe lead pins 106, while increasing the diameters R of the fixingmaterials 107. However, the size (diameter t) of the stem 102 isspecified in accordance with an optical connector to be coupled.Therefore, even if the diameters R of the fixing materials 107 areincreased relative to the diameter t of the stem 102 in such a manner asto establish an impedance matching with external electronic circuitcomponents implemented on the BOARD with the aim of improving the highfrequency characteristic, then the sizes of the holes 102 a to be formedin the stem 102 would be increased, resulting in a degradation of themechanical strength of the stem 102, which would not be practical.

A communication module is known which has, instead of the structurecalled CAN type package, a so-called butterfly structure in which a PD,a semiconductor laser (LD) and so on are directly mounted on a circuitboard on which high frequency lines, such as micro strip lines or thelike, are formed. This module can provide a precise impedance matchingwith external electronic circuit components, but it is larger in sizethan one having the so-called CAN type package structure comprising astem and a cap and hence is not appropriate in a case where a smallersize of communication module is desired.

It is an object of the present invention to provide a communicationmodule that is smaller in size but capable of providing a high-ratecommunication with an excellent high frequency characteristic.

Means for Solving Problem

The present invention achieves the above-described object by employing apackage structure, which comprises a stem and a cap, and further usingno lead pins but using a flexible printed circuit board.

A communication module according to the present invention comprises asemiconductor member; a flexible printed circuit board on which thesemiconductor member is mounted and to which the semiconductor member iselectrically connected; a stem through which the flexible printedcircuit board is inserted and to which the flexible printed circuitboard is then fixed; and a cap so disposed as to cover the semiconductormember.

The communication module of the present invention having the structuredescribed above employs, as members for supplying a power, derivingelectric signals and so on, not lead pins but the flexible printedcircuit board, so that the communication module is not affected by thelead pin lengths and the sizes of the fixing materials used for fixingthe lead pins to the stem and hence can exhibit an improved highfrequency characteristic. Further, since the communication module of thepresent invention employs the flexible printed circuit board that allowsan impedance matching with the external electronic circuit components tobe precisely established, it can have not a butterfly structure but apackage structure comprising a stem and a cap, so that the size of thecommunication module can be further reduced. Thus, the communicationmodule of the present invention is smaller in size but can providecommunications of higher rates than 100 Mbps, particularly, than 1 Gbps.

Moreover, the communication module of the present invention employs theflexible printed circuit board to connect the semiconductor member tothe BOARD, so that no short circuit occurs due to an accidental mutualcontact of flexible printed circuit boards, or due to an accidentalcontact of the flexible printed circuit board with metallic dust, ametallic seal provided along the periphery of the package, or the like.The lead pins of the conventional communication modules as shown inFIGS. 6 and 7 are usually made of a highly conductive metal, such ascopper, aluminum and so on, and the surfaces of such metals, except theportions thereof contacting with the fixing materials used for fixingthem to the stem, are exposed. Accordingly, there is a fear thatmetallic dust, a metallic seal or the like accidentally contacts withlead pins connected to the BOARD and residing between the stem and theBOARD, causing those lead pins to be electrically connected to eachother, resulting in occurrence of a short circuit. Moreover, all theportions of the lead pins, connected to the BOARD, except their portionsfixed to the stem and to the BOARD, that is, those portions of the leadpins which reside between the stem and the BOARD can move to somedegree. Accordingly, there is a fear that those movable portionsaccidentally contact with each other, resulting in occurrence of a shortcircuit. On the other hand, the flexible printed circuit board usuallyhas a structure in which an insulating member (i.e., a cover lay)overlie the entire surfaces of the flexible printed circuit board exceptparticular portions thereof, for example, those where semiconductormembers or the like are mounted. Thus, even if flexible printed circuitboards accidentally contact with each other or with metallic dust, ametallic seal or the like, it will cause no short circuits. Therefore,the communication module of the present invention can prevent anydamages from occurring due to such accidental short circuits. Theprevent invention will be described below in greater detail.

A semiconductor member included in the communication module of thepresent invention may be a light emitting element when the communicationmodule is an optical transmitter module. The light emitting element is,for example, a semiconductor laser (LD), a light emitting diode (LED) orthe like, which may be made of AlGaAs system or InGaAsP system. When thecommunication module of the present invention is an optical receivermodule, a semiconductor member included in the communication module maybe a light receiving element. The light receiving element is, forexample, a photodiode (PD), an avalanche photodiode (APD) or the like,which may be made of InGaAs system, InGaAsP system, Si, Ge or the like.Specifically, in a case of using a light receiving layer that receiveslong-wavelength bands, such as bands of wavelengths from 1 μm to 1.6 μm,the light receiving element is preferably made of InGaAs system, InGaAsPsystem or Ge. In a case of using a light receiving layer that receivesshorter-wavelength bands, the light receiving element may be made of Sior the like. As the light receiving element to be used,front-illuminated type photodiode is preferable because it is easy toimplement. When the communication module of the present invention is anoptical transmitter/receiver module, it may include the same number oflight emitting elements and light receiving elements described above. Inany one of the above-described cases, when the communication module ofthe present invention is a communication module of multi-channel havinga plurality of optical transmission media, such as optical fiber and thelike, it may include a plurality of light emitting elements and lightreceiving elements in accordance with the number of the opticaltransmission media used. The semiconductor member may include variouselectronic elements used for communications and may be an integratedcircuit (IC) in which such elements are electrically connected. Forexample, at the receiving end, such an integrated circuit may be anamplifier for amplifying the output electric power of the lightreceiving element, which is typically a preamplifier IC or a limitingamplifier IC. At the transmitting end, such an integrated circuit may bea driving element, such as a driver IC for driving the light emittingelement.

The semiconductor member described above is mounted on a flexibleprinted circuit board. This flexible printed circuit board may have atypical structure in which one or more layers of wiring patternscomprising conductors, such as copper films or the like, are formed on asurface of, or the surface of and within an insulative basic materialmade of resin, such as polyimide, polyester or the like, and in which aninsulative cover made of resin, such as polyimide, polyester or thelike, is formed over the surfaces of that basic material.

The flexible printed circuit board may include at least one wiringpattern, and the number of such wiring patterns may be appropriatelymodified in accordance with the number of semiconductor members to beelectrically connected to the flexible printed circuit board. A singleflexible printed circuit board may have a plurality of wiring patternsformed therein, which may be connected to respective differentsemiconductor members. For example, in a case of a transmitter/receivermodule, a single flexible printed circuit board may include differentwiring patterns used for respective ones of light emitting and receivingelements. As another example, in a case of a transmitter module, asingle flexible printed circuit board may include a wiring pattern usedfor a light emitting element and a wiring pattern used for a monitoringlight receiving element that can determine the strength of a lightemitted by the light emitting element. In a case of a receiver module, asingle flexible printed circuit board may include a wiring pattern usedfor a light receiving element and a wiring pattern used for an amplifierthat amplifies an electric signal outputted by the light receivingelement. Thus, a plurality of semiconductor members may be mounted on asingle flexible printed circuit board, so that there is no need to forma plurality of fixing holes in the stem as conventionally done. Thisreduction of the number of fixing holes can contribute to an improvementof the strength of the stem.

The flexible printed circuit board is inserted through the stem and thenfixed thereto. Specifically, the flexible printed circuit board may befixed to the stem with one of its ends protruding toward a side wherethe cap is located (which will be referred to as “cap side” hereinafter)and the other end protruding toward the opposite side (which will bereferred to as “BOARD connection side” hereinafter). As another example,the flexible printed circuit board may be bent in such a manner that itsbent portion protrudes from the stem toward the cap side, while its twoends protrude from the stem toward the BOARD connection side. When theflexible printed circuit board is fixed to the stem, a fixing material,such as solder or a glass of a low melting point, may be used. A fixingmaterial may be appropriately selected which has a lower melting pointthan the material used to constitute the flexible printed circuit board,with the heat-resistance thereof taken into account. For example, whenthe flexible printed circuit board is made of polyimide, a fixingmaterial whose melting point is on the order of 300 to 350 degreescentigrade may be appropriately used.

A portion of the flexible printed circuit board protracting from thestem toward the cap side will possibly bend by itself if it is left asit is. Then, a supporting member or the like is preferably provided tothe stem. If the flexible printed circuit board is intentionally bentand disposed, a supporting member is preferably disposed inside such abent portion of the flexible printed circuit board. Provision of such asupporting member can prevent the flexible printed circuit board frombending by itself even when a semiconductor member is mounted thereon,which facilitates the positioning relative to an optical transmissionmedium or the like. In addition, the provision of such a supportingmember can reinforce the flexible printed circuit board. A material usedto constitute the supporting member may be, for example, iron, such ascold-rolled steel sheet (SPC). The supporting member may be formedindependently of the stem and fixed thereto by use of solder or thelike. Alternatively, the supporting member may be integrally formed withthe stem.

As described above, the number of wiring patterns to be formed in asingle flexible printed circuit board may be increased so as to increasethe number of components to be mounted on the single flexible printedcircuit board. Alternatively, a plurality of different flexible printedcircuit boards may be fixed to the stem such that differentsemiconductor members are mounted on the respective flexible printedcircuit boards or such that the flexible printed circuit boards are usedfor different purposes with respect to a single semiconductor member. Inthe latter case, for example, a flexible printed circuit board used fora signal line of a semiconductor member may be differentiated from aflexible printed circuit board used for a ground line (GND line) or DCpower supply line of that semiconductor member. In such a case,necessary wiring patterns have been formed in such flexible printedcircuit boards. For example, the wiring pattern used for the GND line orDC power supply line has been preferably formed in such a manner thatits line width is large. In a case when there is a fear that noisepossibly occurs between signal lines if included in the same singleflexible printed circuit board or between a signal line and a powersupply line if included in the same single flexible printed circuitboard, it is also preferable to employ a plurality of flexible printedcircuit boards for respective different purposes.

Although the shape of the flexible printed circuit board to be employedis not specified, yet it is preferable that the flexible printed circuitboard is suitably shaped beforehand for where it is to be located,because of ease with which to dispose the flexible printed circuit boardthere. For example, the possible shapes of the flexible printed circuitboard when in a plane may include bent-shapes; specifically, L-shapes,S-shapes and so on.

At least a part of wiring patterns formed in the flexible printedcircuit board is preferably a transmission line that can be used for ahigh frequency band. Such transmission lines may be a type oftransmission lines selected from among, for example, coplanar lines,micro-strip lines and grounded coplanar lines. These lines can be formedby use of any known method.

The flexible printed circuit board is connected to a BOARD (i.e., asubsequent-stage circuit board) on which external electronic circuitcomponents and the like are implemented. Although the connection of theflexible printed circuit board to the BOARD may be achieved bysoldering, yet it also may be achieved by providing a connector, whichcan connect to the BOARD, to a BOARD connection side of the flexibleprinted circuit board and then connecting the connector to the BOARD. Inthis case, when electronic circuit components are fixed to the BOARD byreflow soldering, the connector can be also fixed thereto at the sametime, with the result that the workability of assembly is improved.Moreover, in a case of connecting, by manual soldering, the flexibleprinted circuit board to the BOARD, if a semiconductor member, theflexible printed circuit board or the like suffers any damage, then notonly it but also the BOARD must be replaced. In a case of using theconnector to connect the flexible printed circuit board to the BOARD,only a removal of the connector would allow the BOARD to be reused,advantageously resulting in a cost reduction.

The communication module of the present invention has a packagestructure comprising a stem and a cap. The flexible printed circuitboard is fixed to the stem. The cap is so disposed on the stem as tocover a semiconductor member mounted on the flexible printed circuitboard that is fixed to the stem after having been inserted therethrough.The stem and cap are preferably made of metallic materials, such as iron(Fe), copper (Cu), or copper-nickel alloy (Cu—Ni), or iron alloy, forexample, SPC, stainless, Fe—Co—Ni or the like. The package made of suchmetallic materials has a strong structure, exhibits an excellentlong-term stability because of hermetic seal, also exhibits an excellentheat radiation, and also has a function of eliminating externalelectromagnetic noise.

In a case when the communication module includes a light emittingelement or a light receiving element, the cap is preferably soconstructed as to have therein a light-collective lens capable ofcoupling a light between a light transmission medium and the lightemitting or receiving element, for the purpose of improving theworkability of assembly. This lens may be any one that only can transmittherethrough the wavelength of a light from the light emitting elementor to the light receiving element, and that may be made of a glass, forexample, BK-7 (trade name) available from Schott, or the like.

As previously described, the communication module of the presentinvention may include, in addition to a light emitting element and/or alight receiving element, a monitoring light receiving element, anamplifier, a driving element and so on. The amplifier and drivingelement may be, for example, Si-IC, GaAs-IC or the like. In a case whenthe communication module includes an amplifier in addition to a lightreceiving element, the amplifier is preferably disposed in the vicinityof the light receiving element so that the length of a metallic wire,such as gold (Au), aluminum (Al) or the like, for connecting theamplifier to the light receiving element can be reduced, therebyenhancing the resistance to noise. As the foregoing monitoring lightreceiving element, one may be used which is similar to the lightreceiving element as described above.

EFFECT OF THE INVENTION

As described above, the communication module of the present inventionhas the package structure, which comprises the stem and the cap, andemploys, as members for supplying a power to a semiconductor member,deriving signals therefrom and so on, not lead pins but flexible printedcircuit boards, thereby providing an excellent advantage that the highfrequency characteristic can be improved regardless of the lengths ofthe lead pins and the sizes of the fixing materials used for fixing tothe stem. Accordingly, the communication module of the present inventioncan be used for providing communications of higher rates than 100 Mbps,particularly, than 1 Gbps. Because of employing the flexible printedcircuit board, the communication module of the present invention canprovide a more precise impedance matching with external electroniccircuit components, and yet it is smaller in size than the conventionalone because of employing the package structure as described above.

MODES FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described below.

EMBODIMENT 1

FIG. 1 is a schematic structure diagram illustrating an example of thecommunication module of the present invention having a light emittingelement. A communication module 1 of the present embodiment includes anLD 10; a flexible printed circuit board (which will be referred to as“FPC” hereinafter) 11 on which the LD 10 is mounted and to which the LD10 is electrically connected; a stem 12 through which the FPC 11 isinserted and to which the FPC 11 is then fixed; and a cap 13 so disposedas to cover the LD 10. Their structures will be described below ingreater detail.

The LD 10 is a semiconductor device that emits a light, which isincident upon an optical fiber 200. In the present embodiment, the LD 10is made of InGaAsP system. It should be noted that in the presentembodiment, an excellent LD, which has been tested in advance, is usedas the LD 10, so that deficiencies can be reduced.

The FPC 11 is a member on which the LD 10 is mounted and which iselectrically connected to the LD 10 to supply a power thereto, derivesignals therefrom and so on. In the present embodiment, the FPC 11 has astructure comprising an internal layer part 11 a, which comprises a basematerial made of polyimide and which has, on and within the basematerial, wiring patterns comprising copper films, and insulative covers11 b made of polyimide and overlying the two surfaces of the internallayer part 11 a. The FPC 11 is inserted through a fixing hole 12 aformed in the stem 12 and then fixed thereto by use of a fixing material15 with one of its ends protruding toward the cap 13 (the upper side ofFIG. 1) and the other end protruding toward the BOARD (not shown) (thelower side of FIG. 1). In the present embodiment, a glass of a lowmelting point (300 degrees centigrade) is used as the fixing material15.

A portion of the FPC 11, which protrudes toward the cap 13, bears the LD10, which is electrically connected to a wiring pattern formed in theFPC 11. In the present embodiment, a part 11 c of the FPC 11 on whichthe LD 10 is mounted has been rust-proofed and plated, and then asolder, the melting point of which is 300 degrees centigrade, has beenused to fix the LD 10 to the part 11 c. A bonding wire 14 made of goldis used to connect the LD 10 to the FPC 11. In the present embodiment,the wiring pattern to which the LD 10 is connected is of micro striplines.

In the present embodiment, the stem 12 has a supporting member 16 forsupporting the portion of the FPC 11 protruding toward the cap 13 so asto prevent that portion from bending by itself. The supporting member 16may be anything that can support the portion of the FPC 11 protrudingtoward the cap 13 to prevent that portion from bending by itself. Thesupporting member 16 is a block of SPC and is fixed to the stem 12 byuse of a solder in the present embodiment, but, alternatively, may beintegrally formed with the stem 12.

A combination of the stem 12 and the cap 13 serves as a package forprotecting the LD 10. The present embodiment employs, as this package,one which is made of stainless that exhibits an excellentmechanical-strength and an excellent heat radiation, can be hermeticallysealed, and that has a function of eliminating electromagnetic noise. Inthe present embodiment, the central axis of the package is coaxial withthe optical axis of the optical fiber 200. As described above, the FPC11 is inserted through and then fixed to the hole 12 a of the stem 12 byuse of the fixing material 15. The cap 13 has a light-collective lens 13a for allowing a light from the LD 10 to be optically coupled to theoptical fiber 200 with a high degree of efficiency. The light-collectivelens 13 a is so disposed that its central axis is coaxial with theoptical axis of the optical fiber 200. The structures of the stem 12,cap 13 and light-collective lens 13 a are the same as those inembodiments 2 and 3 that will be described later.

The present embodiment provides, on the stem 12, a monitoring PD 17 thatcan determine the strength of a light emitted from the LD 10. In thepresent embodiment, the monitoring PD 17, which is made of InGaAssystem, is front-illuminated type photodiode, and is located under theLD 10 as shown in FIG. 1. The monitoring PD 17 is connected to the FPC11 by use of a bonding wire 14.

The communication module of the present invention constructed asdescribed above, though having the package structure of so-called CANtype, can exhibit an improved high frequency characteristic, regardlessof the lead pin lengths and the sizes of the fixing materials, becauseof employing, as a member for supplying a power to the semiconductordevice, deriving electric signals therefrom and so on, the FPC insteadof lead pins. Further, because of employing the package structure of CANtype, the communication module of the present invention can beconstructed in a smaller size than the conventional one having a blockstructure. In particular, the communication module of the presentembodiment has, as wiring patterns formed in the FPC, transmission linesexhibiting an excellent high frequency characteristic, so that theimpedance matching with the external electronic circuit components canbe precisely established.

EMBODIMENT 2

The foregoing embodiment was described as to a transmitter module havinga light emitting element. The communication module of the presentinvention may be a receiver module having a light receiving element.FIG. 2 (A) is a schematic structure diagram illustrating an example ofthe communication module of the present invention having a lightreceiving element, and FIG. 2 (B) is a schematic diagram illustrating amagnified view of an FPC. Elements and members that are the same asthose illustrated in FIG. 1 are designated by the same referencenumbers. A communication module 2 of the present embodiment includes aPD 20; an FPC 11A on which the PD 20 is mounted and to which the PD 20is electrically connected; an FPC 11B to which the PD 20 is electricallyconnected; a stem 12 through which the FPCs 11A and 11B are inserted andto which they are then fixed; and a cap 13 so disposed as to cover thePD 20. Their structures will be described below in greater detail.

The PD 20 is a semiconductor device that receives a light incident froman optical fiber 200. In the present embodiment, the PD 20, which ismade of InGaAs, is front-illuminated type photodiode. It should be notedthat in the present embodiment, an excellent PD, which has been testedin advance, is used as the PD 20, so that deficiencies can be reduced.

The present embodiment employs a plurality of FPCs each having a similarstructure to the one of the FPC employed by the foregoing embodiment 1,and these FPCs 11A and 11B are fixed to the stem 12 by use of fixingmaterials 15. FIG. 2 shows two FPCs 11A and 11B, one of which, FPC 11B,is fixed to the stem 12 with one of its ends protruding toward the cap13 and the other end protruding toward the BOARD (not shown) in asimilar manner to the foregoing embodiment 1. The other FPC 11A isinserted through a hole 12 a toward the cap 13, thereafter bent back,inserted again through another hole 12 a, and then fixed to the stem 12.That is, as shown in FIG. 2 (A), the bent portion of the FPC 11Aprotrudes toward the cap 13, while the two ends thereof protrude towardthe BOARD (not shown). As shown in FIG. 2 (B), the FPC 11A of thepresent embodiment is so constructed as to have an inner layer part 11a, which comprises a base material 22 made of polyimide and which has,on the two surfaces of and within the base material 22, a plurality ofwiring patterns 23, and insulative covers 11 b overlying the twosurfaces of the inner layer part 11 a. The PD 20 and a preamplifier IC21, which amplifies an output from the PD 20, are fixed to respectivedifferent ones of the wiring patterns 23 of the FPC 11A by use of solderlid, the melting point of which is 300 degrees centigrade. It should benoted that parts 11 c of the FPC 11A on which the PD 20 and preamplifierIC 21 are mounted have been rust-proofed and plated. Bonding wires 14made of gold are used to provide electrical connections between the PD20 and the FPC 11A and between the preamplifier IC 21 and the FPC 11A.In the present embodiment, the wiring pattern to which the PD 20 isconnected and the wiring pattern to which the preamplifier IC 21 isconnected are of coplanar lines. The preamplifier IC 21, which comprisesSi-IC, is disposed in the vicinity of the PD 20 so as to reduce thelength of a wire (not shown) which connects the preamplifier IC 21 tothe PD 20, thereby reducing the affection of noise.

In the present embodiment, the FPC 11B is used as power supply lines forthe PD 20 and preamplifier IC 21, and has wiring patterns that arelarger in line width than those of the FPC 11A. Bonding wires made ofgold are used to provide connections between the PD 20 and the FPC 11Band between the preamplifier IC 21 and the FPC 11B. (The bonding wireused for the latter connection is not shown.)

Similarly to the foregoing embodiment 1, the present embodiment employsa supporting member 16A, disposed inside the bent portion of the FPC 11Aprotruding toward the cap 13, for supporting the bent portion so as toprevent it from bending by itself. The supporting member 16A is a blockof SPC and is fixed to the stem 12 by use of solder.

As described above, the communication module of the present inventionmay include a plurality of FPCs and a plurality of semiconductormembers. Similarly to the foregoing embodiment 1, the presentcommunication module can exhibit an improved high frequencycharacteristic and also can be constructed in a smaller size. Moreover,the communication module of the present embodiment has, as the wiringpatterns formed in the FPC, transmission lines having an excellent highfrequency characteristic, whereby the impedance matching with theexternal electronic circuit components can be readily established.

It should be noted that though the PD 20 and preamplifier IC 21 both aremounted on the FPC 11A in the present embodiment, they may be mounted onrespective different FPCs. Specifically, the PD 20 may be mounted on theFPC 11A, while the preamplifier IC 21 may be mounted on the FPC 11B.

EMBODIMENT 3

The foregoing embodiments 1 and 2 were described as to the transmittermodule and the receiver module. The communication module of the presentinvention may be a transmitter/receiver module having both a lightemitting element and a light receiving element. FIG. 3 is a schematicstructure diagram illustrating an example of the communication module ofthe present invention having light emitting and receiving elements.Elements and members that are the same as those illustrated in FIGS. 1and 2 are designated by the same reference numbers. A communicationmodule 3 of the present embodiment includes an LD 10; an FPC 11C onwhich the LD 10 is mounted and to which the LD 10 is electricallyconnected; a PD 20; an FPC 11D on which the PD 20 is mounted and towhich the PD 20 is electrically connected; a stem 12 through which theFPCs 11C and 11D are inserted and to which they are then fixed; and acap 13 so disposed as to cover the LD 10 and PD 20. Their structureswill be described below in greater detail.

The present embodiment employs two FPCs having similar structures to theones of the FPCs employed by the foregoing embodiment 2. Each of the twoFPCs is fixed to the stem 12 with one of its ends protruding toward thecap 13 and the other end protruding toward the BOARD (not shown)similarly to the foregoing embodiment 1. One of the two FPCs, namely FPC11C, bears the LD 10 for transmission, while the other, namely FPC 11D,bears the PD 20 for reception. The present embodiment employs an opticalpath conversion part 30 capable of focusing and separating a lightincident from the LD 10 upon an optical fiber 200 and a light outgoingfrom the optical fiber 200 toward the PD 20.

The optical path conversion part 30, which has a WDM (wavelengthdivision multiplex) filtering function, includes atransmission/reflection part 30 a that allows one of the incident andoutgoing lights as described above to pass therethrough and reflects theother light. The transmission/reflection part 30 a may be formed byusing PVD or CVD methods to form a film over a surface of a substratemade of a transparent glass or the like. As to the materials of the filmformed, the film comprises a multi-layered film of dielectric materials,which may alternately have a film of a low-refractive material, such asSiO₂, MgF₂ or the like, and a film of a high-refractive material, suchas Al₂O₃, Ti₂O₅ or the like. In the present embodiment, a plasma CVDmethod (P-CVD method) is used to alternately deposit films of SiO₂ andTi₂O₅ over a transparent glass substrate. The optical path conversionpart 30 is fixed to a supporting member 16 that supports the FPC 11C. Itshould be noted that the incident and outgoing lights as described abovehave been caused to exhibit mutually different wavelengths. In thepresent embodiment, the former exhibits a wavelength of 1.3 μm and thelatter exhibits a wavelength of 1.55 μm.

The LD 10 and PD 20, which are the same as those in the foregoingembodiments 1 and 2, are connected to the FPCs 11C and 1D, respectively,by use of bonding wires 14 made of gold. Supporting members 16 thatsupport the FPCs 11C and 11D are integrally formed with the stem 12. Thewiring pattern to which the LD 10 is connected and that to which the PD20 is connected are of grounded coplanar lines. Additionally, in thepresent embodiment, the package includes a monitoring PD 17 and apreamplifier IC (not shown) that amplifies an output from the PD 20, andit may further include an IC for driving the LD 10, and the like.

As described above, the communication module of the present inventionmay be a transmitter/receiver module. Because of employing not lead pinsbut FPCs, this communication module can exhibit an improved highfrequency characteristic and also can be constructed in a smaller sizesimilarly to the foregoing embodiments 1 and 2. Moreover, thecommunication module of the present embodiment has, as the wiringpatterns formed in the FPCs, transmission lines having an excellent highfrequency characteristic, whereby the impedance matching with theexternal electronic circuit components can be readily established.

The foregoing embodiments 1 through 3 were described as having a lightemitting element and/or a light receiving element, but as a matter ofcourse, they may have a structure that includes only integrated circuits(ICs). In such cases, the cap may have no light-collective lens.

EMBODIMENT 4

In the foregoing embodiments 1 through 3, solder or the like is used toconnect the FPCs to the BOARD. Next, a structure that facilitates theconnection to the BOARD will now be described. FIG. 4 is a schematicdiagram illustrating a communication module of the present inventionhaving a connector. Elements and members that are the same as thoseillustrated in FIG. 1 are designated by the same reference numbers. Acommunication module 4 of the present embodiment is the same as thecommunication module of the embodiment 1 in the basic structure butdifferent in that it employs a connector 40 that is provided to a BOARDconnection end of an FPC 11 and that can connect to the BOARD.

Because of the structure having the connector, when electronic circuitcomponents are fixed to the BOARD by use of reflow soldering, theconnector can be also fixed at the same time, whereby the workability ofassembly is improved. Moreover, the connector is removable from theBOARD, and hence, if any trouble occurs in a constituent element, suchas a semiconductor member or FPC, then the connector is removed, wherebythe BOARD can be reused.

EMBODIMENT 5

In the foregoing embodiments 1 through 4, examples employing FPCs havingrectangular shapes when in a plane were described, but, as a matter ofcourse, FPCs having different shapes may be employed. FIG. 5 is aschematic diagram illustrating a communication module of the presentinvention employing an FPC that is L-shaped when in a plane. Elementsand members that are the same as those illustrated in FIG. 1 aredesignated by the same reference numbers. A communication module 5 ofthe present embodiment is the same as the communication module of theembodiment 1 in the basic structure but different in that an FPC 50 hasa bent-shape when in a plane.

As described above, the communication module of the present inventionmay employ modified shapes of FPCs. Thus, the FPCs can be suitablyshaped beforehand for where they are to be located. The presentembodiment employs the FPC that is bent in an L-shape when in a plane,but, as a matter of course, any other shapes, such as S-shapes and soon, of FPCs may be employed.

INDUSTRIAL APPLICABILITY

The communication module of the present invention is used for opticalcommunications, and in particular, the most suitable for usages thatrequire an excellent high frequency characteristic and a high ratecommunication.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic structure diagram illustrating an example of thecommunication module of the present invention having a light emittingelement.

FIG. 2 (A) is a schematic structure diagram illustrating an example ofthe communication module of the present invention having a lightreceiving element. FIG. 2 (B) is a schematic diagram illustrating amagnified view of an FPC.

FIG. 3 is a schematic structure diagram illustrating an example of thecommunication module of the present invention having light emitting andreceiving elements.

FIG. 4 is a schematic diagram illustrating a communication module of thepresent invention having a connector.

FIG. 5 is a schematic diagram illustrating a communication module of thepresent invention employing an FPC that has a bent-shape when in aplane.

FIG. 6 (A) is a front elevational view of the longitudinalcross-sectional structure of a conventional optical receiver module.FIG. 6 (B) is a plan view of a stem as viewed from a BOARD connectionside.

FIG. 7 is a side view showing a state in which the conventional opticalreceiver module is connected to the BOARD.

EXPLANATIONS OF LETTERS OR NUMERALS

-   1,2,3,4,5 Communication module-   10 LD 11,11A,11B,11C,11D,50 FPC-   11 a Internal layer part 11 b Insulative covers-   11 c Part lid Solder 12 Stem 12 a Hole-   13 Cap 13 a Light-collective lens 14 Bonding wire-   15 Fixing material 16,16A Supporting member-   17 Monitoring PD-   20 PD 21 Preamplifier IC 22 Base material-   23 Wiring patterns-   30 Optical path conversion part-   30 a Transmission/Reflection part 40 Connector-   100 Optical receiver module 101 PD-   102 Stem 102 a Hole 103 Light-collective lens-   104 Cap 105 Sub-mount 106 Lead pin-   107 Fixing material 108,112 Wire 109 Preamplifier-   110 Board 111 wiring pattern-   200 Optical fiber

1. A communication module comprising: a semiconductor member; a flexibleprinted circuit board on which said semiconductor member is mounted andto which said semiconductor member is electrically connected; a stemthrough which said board is inserted and to which said board is thenfixed; and a cap so disposed as to cover said semiconductor member. 2.The communication module according to claim 1, wherein the semiconductormember is at least one of a light emitting element, a light receivingelement and an integrated circuit.
 3. The communication module accordingto claim 1, wherein the flexible printed circuit board includes a typeof lines selected from among coplanar lines, micro-strip lines, andgrounded coplanar lines.
 4. The communication module according to claim1, wherein a plurality of different flexible printed circuit boards arefixed to the stem.
 5. The communication module according to claim 1,wherein an end of the flexible printed circuit board(s) that protrudesfrom the stem has a connector that can connect to a subsequent-stagecircuit board.
 6. The communication module according to claim 1, whereinthe flexible printed circuit board(s) exhibits a bent-shape when in aplane.